Power transmission and distribution grids transmit electrical energy from generating facilities to end users. Voltage management on the transmission and distribution system is an important consideration for the operational and design of the system. In a typical system reactive power flow has a strong influence on voltage. Reactive power flow can be influenced by the generator source, changes in the transmission and distribution system, the addition of shunt reactive elements, and loads. Furthermore, excessive reactive power flow can raise voltage and put undue stress on transmission lines, transformers and other electrical components.
With reference to FIGS. 1, 2 and 3, electrical power has at least two characteristics relevant to power distribution: voltage and current. In a large-scale power distribution grid both voltage and current vary over time. When the instantaneous voltage is multiplied by the instantaneous current, the result is the instantaneous power. In most power distribution grids the voltage and current signals have the form of a sine wave.
If the reactive power (i.e., VAR) flow is zero, the voltage and current waves are in phase as illustrated in FIG. 1, where ν(ωt) is the time-varying voltage wave form and i(ωt) is the time-varying current wave form. However, if the reactive power (i.e., inductive or capacitive) is non-zero, the voltage wave form, ν(ωt), will not be in phase with the current wave form, i(ωt). The amount by which the current lags or leads the voltage can be quantified by a power factor angle, φ, which is representative of the fraction of a cycle by which the current leads or lags the voltage. A cycle is 2π or 360°, and the power factor angle, φ, is the difference between the cycles of the current and voltage.
With respect to a constant voltage wave form, ν(ωt), a lagging current is illustrated as i(ωτ−φ) in FIG. 2 and a leading current is illustrated as i(ωτ+φ) in FIG. 3. The amount by which the current lags or leads the voltage can be quantified by a power factor angle φ, which is representative of the fraction of a cycle by which the current lags or leads the voltage. A cycle is 2π or 360°, and the power factor angle, φ, is the difference between the cycles of the current and the voltage.
Reactive power factor is important from the standpoint of power delivery. Since most transmission systems are inductive, increasing the reactive current component (i.e., capacitive VARs) will cause the voltage to rise. Conversely, decreasing the reactive power component (i.e., inductive VARs) will cause the voltage to decrease.
Wind farm reactive power flow control can be achieved by the individual wind turbine generator, shunt elements (e.g., switched capacitors or switched reactors), transformer tap changers, or some combination of these.